Fouling tests to define critical conditions in dead end membrane processes: towards sustainable operating conditions
Résumé
Ultrafiltration, which is currently used as the principal step for potable water production, is still limited by fouling. Fouling causes an additional hydraulic resistance leading to higher energy consumption and therefore to an increase in operating costs. In order to improve productivity and water quality, Aquasource (Toulouse, France) has decided to develop a new water production unit using exclusively a dead-end process. The optimisation of such a unit requires a better understanding of fouling phenomena occurring during a dead-end filtration in order to find the most suitable association between membrane and running parameters according to raw water characteristics. In this way, a succession of filtration step and gentle rinsing phase is used to quantify the residual fouling resistance remaining after the rinse. When processed for different filtered volumes between the rinsing steps, this method determines a critical filtered volume, above which the residual resistance increases drastically, which seems to be an equivalent concept to the critical flux for dead-end filtration. This protocol can then leads to a first depict of fouling contributions in terms of reversible and irreversible contributions where the reversible part can be attributed to concentration polarization whereas the irreversible fouling can be assigned to deposit formation and/or adsorption. In a second part of this work, with intent to go thoroughly into these experimental results, the influence of different fractions of raw water on the fouling rate and on the fouling irreversibility is analysed through the comparison of filtration of the raw water (canal du midi, Toulouse, France) and the same ultrafiltered water and their associated backwashes efficiencies. This kind of experiment allows the differentiation of particulate and dissolved behaviour relative to cake and adsorption fouling mechanisms. These two tests presented here allow us to split up the fouling contributions: concentration polarization and deposit formation can be depicted by the critical filtered volume concept, whereas backwashes efficiency gives information about adsorption and therefore on water classification. Furthermore, the critical filtered volume determination makes it then possible to fix operating conditions leading to a sustainable operation where the use of membrane cleaning is limited.